Thermoplastic polymers are commonly used to manufacture various shaped articles that may be use in applications such as automotive parts, food containers, electrical parts, etc. Shaped articles comprising polyester may be prepared from the molten polymer by a number of melt extrusion processes such as injection molding, compression molding, and blow molding.
Poly(trimethylene) terephthalate (PTT), produced from renewable source of 1,3-propanediol and developed by E. I. du Pont de Nemours & Company (DuPont), is commercially available under Sorona® resin. DuPont pioneered a way to produce the 1,3-propanediol from renewable resources including corn sugar. Sorona® resin has properties including semi-crystalline molecular structure.
It is desirable to develop a thermoplastic polyester composition such as PTT that can be crystallized rapidly from the melt, in order to improve the injection molding processing, and at the same time, improve the hydrolytic stability of the thermoplastic composition.
Furthermore, PTT has a higher equilibrium cyclic oligomer concentration, typically about 2.5% by weight based on the resin weight, when compared to similar polyesters such as PET or PBT, that typically have 1.4-1.8% by weight of cyclic oligomer. The most abundant cyclic oligomer of PTT is the cyclic dimer. When PTT resin molded parts are subjected to higher than normal temperature conditions (80° C. to 160° C.) the cyclic dimer of PTT is observed to bloom to the surface of the molded part, resulting in an undesirable cosmetic defect. The surface of a black part (containing carbon black) whitens with a crystalline powder of cyclic dimer. A related problem for polymer compositions, in particular polyester compositions and polymer compositions having a polyester component, is the release of low molecular weight components when heated, referred to as “outgassing”. This can be a particular problem in polymer parts that are often or even constantly subjected to high temperatures, such as a bezel, a housing for a lamp or a reflector for a lamp, all of which are heated by the lamp.
U.S. Pat. No. 6,441,129, Duh, et al, discloses a process for producing PTT at an increased solid state polymerization rate. The concentration of cyclic oligomer in the PTT provided by the process is not disclosed. Duh also discloses specific solid state polymerization processes in J. Appl. Polymer Sci., Vol 89, 3188-3200 (2003).
U.S. Pat. No. 7,332,561 discloses a PTT composition in the form of fine particles having a cyclic dimer content of 1.5% by weight or less, and a process fro making the composition.